The precise patterning of the developing skeletal framework relies on the appropriate control of chondrogenesis, a multistep process during which mesenchymal cells differentiate into chondrocytes. Articular cartilage destruction is a central factor in the pathogenesis of joint diseases, including osteoarthritis (OA) and rheumatoid arthritis (RA). Research on the molecular mechanisms of chondrogenesis is of developmental biological interest and has the potential to lead to novel approaches in tissue engineering and novel arthritis treatments. MicroRNAs (miRNAs) are a family of ~22-nucleotide (nt) noncoding RNAs that regulate gene expression by posttranscriptional mechanisms. To date, chondrocyte specific miRNAs, their upstream molecular signals, and target genes are largely unknown. In our preliminary experiments, we performed microarray profiling of miRNA expression in chondrocytes and miR-140 expression in cartilage development during embryogenesis and adult cartilage chondrocytes. These studies demonstrated that miR-140 has the most cartilage specific expression pattern. Cartilage specific Sox9 null mice did not express miR-140 and the potential Sox9 binding region close to pre-miR-140 was associated with Sox9 by ChIP assay and EMSA, suggesting that Sox9 is a key regulator of miR-140 gene expression. miR-140 expression was down-regulated in osteoarthritic cartilage compared with normal cartilage and by IL-1 stimulation of chondrocytes. miR-140 null mice showed earlier onset of osteoarthritic changes in knee joint at 6 months. Microarray and bioinformatics analysis revealed a set of possible miR-140 target genes in chondrocytes, including ADAMTS-5, which is a key tissue-degrading enzyme in osteoarthritis. These observations support the hypothesis that miR-140 is a novel regulator of cartilage development and homeostasis and those changes in its expression and function play an important role in diseases affecting articular cartilage, such as osteoarthritis. We propose the following specific aims: Aim 1: Investigate the transcriptional regulation of miR-140 expression by Sox9 in mesenchymal stem cells and chondrocytes. Aim 2: Examine the role of miR-140 in chondrogenesis and cartilage development. Aim 3: Determine the expression and function of miR-140 in osteoarthritis. Aim 4: Identify novel target genes of miR-140 in chondrocytes and mesenchymal stem cells. The proposed studies have the potential to reveal important new regulatory pathways that control cartilage development and homeostasis and open new insight on disease mechanisms and therapeutic interventions.